This invention relates to a method of introducing impurity species into a semiconductor structure and more particularly to the diffusion of impurity species from a thin film source deposited on a semiconductor structure.
It has been recognized that impurity diffusion into III-V compound semiconductors is an important step in the fabrication of optoelectronic devices. Recently, much attention has been given to the diffusion of Si into GaAs. Also, recently, considerable attention has been given to impurity induced disordering (IID) in GaAs/GaAlAs quantum well structures.
In particular, the diffusion of silicon in GaAs has been under study and investigation for many years, As an example, G.R. Antell in an article, "The Diffusion of Silicon in Gallium Aresenide", Solid-State Electronics, Vol. 8, pp. 943-946 (1965), discloses the diffusion of Si into GaAs carried out at high temperatures in a sealed quartz capsule containing an overpressure of As to prevent the outdiffusion of As from the GaAs. The diffusivity and activation of Si in GaAs is proportional to the As overpressure and the Ga vacancy concentrations. Excess As pressure in a closed ampoule is required for successful diffusion. See, also, the more recent article on this subject of E. Omura et al, "Closed-Tube Diffusion of Silicon in GaAs From Sputtered Silicon Film", Electronic Letters, Vol. 22(9), pp. 496-498 (Apr. 24, 1986).
More recently, the effects of encapsulation relative to Si implanted into GaAs have been studied to prevent the outdiffusion from GaAs and provide, in some cases, a source of Si for diffusion into GaAs. See the article of T. Onuma et al, "Study of Encapsulants for Annealing Si-Implanted GaAs", Journal of Electrochemical Society, Vol. 129(4), pp. 837-840 (April, 1982). Diffusion of Si was enhanced by SiO.sub.2 encapsulation but was negligible with Si.sub.2 N.sub.4 encapsulation or when capless. The activation of the diffusion process is initiated at high anneal temperatures, such a s 750.degree. C. and above. Onuma et al indicates that the SiO.sub.2 cap layer is permeable to Ga while the Si.sub.3 N.sub.4 cap layer is impermeable to Ga and As so that SiO.sub.2 permits the diffusion of Ga to provide for Ga vacancies in the GaAs and the substitution of Si. However, due to the deposition method employed, cracks developed in some of the samples when subjected to the subsequent high temperature annealing process.
M.E. Greiner et al in the article, "Diffusion of Silicon in Gallium Arsenide Using Rapid Thermal Processing: Experiment And Model", Applied Physics Letters, Vol. 44(8), pp. 750-752 (Apr. 15, 1984) examines Si diffusion from a thin elemental deposited source thereof using rapid thermal processing with several different encapsulants. The results show that diffusion was dependent on the type of encapsulant. The Si source layer and the encapsulants were deposited at relative low temperatures, i.e. below 450.degree. C., with subsequent annealing being accomplished at high temperatures of 850.degree. C.-1050.degree. C. High concentrations of Si diffused into GaAs resulted from a SiO.sub.2 capped thin Si source layer. In particular, a model proposed by Greiner et al explains that paired Si atoms can move substitutionally by exchanging sites with either Ga or As vacancies which explains the enhanced diffusion when using a SiO.sub.2 cap.
More recently, K.L. Kavanagh et al in the articles, "Silicon Diffusion at Polycrystalline-Si/GaAs Interfaces", Applied Physics Letters, Vol. 47(11), pp. 1208-1210 (Dec. 1, 1985) and "The Polycrystalline-Si Cintact to GaAs", Journal of the Electrochemical Society, Vol. 133(6), pp. 1176-1179 (June, 1986), reveals that, under proper conditions, the addition of As to the Si source layer revealed further enhanced diffusion, i.e. grater concentrations, of Si into the GaAs. These conditions called for depositing hydrogenated amorphous Si (a-Si:H) onto GaAs in a silane plasma at 450.degree. C. and subsequent annealing at temperatures between 600.degree. C.-1020.degree. C. The results showed that high level interdiffusion of Si atomic pairs with Ga and As vacancies occurs when As is initially added to the Si source layer. However, the surface area of films deposited onto GaAs continued to have a large number of randomly spaced bubbles, indicative of compressive stresses in the film, developed after the high temperature annealing process.
It is an object of this invention to provide a method of introducing impurity species into a semiconductor structure from a deposited impurity source without incurring bubbles or cracks or other irregularities in the film or to the underlying semiconductor structure.